Manufactures of internal combustion engines, particularly those used in automotive applications, must satisfy customer demands for performance while meeting various regulations for reduced exhaust emissions and improved vehicle economy. One example of a method for improving fuel economy is to operate an engine at an air/fuel ratio that is lean (excess oxygen) of stoichiometry. Examples of lean-burn engines include compression ignition (diesel) and lean-burn spark ignition engines. While lean-burn engines may improve fuel economy, the exhaust gas emitted from such an engine, particularly a diesel engine, is a heterogeneous mixture that contains gaseous emissions such as carbon monoxide (“CO”), unburned hydrocarbons (“HC”) and oxides of nitrogen (“NOx”) as well as condensed phase materials (liquids and solids) that may constitute particulate matter (“PM”). The efficient reduction of these exhaust gas constituents is important to meet emission standards and improve vehicle economy.
Several exhaust treatment systems have been proposed for vehicle applications that employ various exhaust treatment devices. One such treatment device employs a Selective Catalyst Reduction (“SCR”) catalyst disposed on a catalyst support and a NOx reductant (e.g. liquid urea) that is injected upstream of the SCR catalyst. The NOx reductant/catalyst combination is effective to reduce the level of NOx in the exhaust gas in a known manner. An exhaust treatment system in use for reducing high levels of PM in the exhaust gas is the particulate filter (“PF”) device. The filter is a physical structure for removing particulates from exhaust gas and, as a result, the accumulated particulates must be periodically removed through a regeneration process that involves increasing the temperature of the filter by injecting fuel directly into the exhaust system in a known manner.
In both of the exhaust treatment systems discussed above, the injected fluid requires a predetermined amount of time in the exhaust gas flow for proper preparation of the mixture (e.g. mixing of, or chemical conversion of, or vaporization of, or distribution of the fluid) before it can properly react to provide the desired benefit. Due to often limited packaging space, it is therefore desirable to provide a system and method that is capable of doing so in the shortest length possible.